Python Dictionary Comprehension

Question

Is it possible to create a dictionary comprehension in Python (for the keys)?

Without list comprehensions, you can use something like this:

l = [] for n in range(1, 11):     l.append(n) 

We can shorten this to a list comprehension: l = [n for n in range(1, 11)].

However, say I want to set a dictionary's keys to the same value. I can do:

d = {} for n in range(1, 11):      d[n] = True # same value for each 

I've tried this:

d = {} d[i for i in range(1, 11)] = True 

However, I get a SyntaxError on the for.

In addition (I don't need this part, but just wondering), can you set a dictionary's keys to a bunch of different values, like this:

d = {} for n in range(1, 11):     d[n] = n 

Is this possible with a dictionary comprehension?

d = {} d[i for i in range(1, 11)] = [x for x in range(1, 11)] 

This also raises a SyntaxError on the for.

Answer 1

There are dictionary comprehensions in Python 2.7+, but they don't work quite the way you're trying. Like a list comprehension, they create a new dictionary; you can't use them to add keys to an existing dictionary. Also, you have to specify the keys and values, although of course you can specify a dummy value if you like.

>>> d = {n: n**2 for n in range(5)} >>> print d {0: 0, 1: 1, 2: 4, 3: 9, 4: 16} 

If you want to set them all to True:

>>> d = {n: True for n in range(5)} >>> print d {0: True, 1: True, 2: True, 3: True, 4: True} 

What you seem to be asking for is a way to set multiple keys at once on an existing dictionary. There's no direct shortcut for that. You can either loop like you already showed, or you could use a dictionary comprehension to create a new dict with the new values, and then do oldDict.update(newDict) to merge the new values into the old dict.

Answer 2

You can use the dict.fromkeys class method ...

>>> dict.fromkeys(range(5), True) {0: True, 1: True, 2: True, 3: True, 4: True} 

This is the fastest way to create a dictionary where all the keys map to the same value.

But do not use this with mutable objects:

d = dict.fromkeys(range(5), []) # {0: [], 1: [], 2: [], 3: [], 4: []} d[1].append(2) # {0: [2], 1: [2], 2: [2], 3: [2], 4: [2]} !!! 

If you don't actually need to initialize all the keys, a defaultdict might be useful as well:

from collections import defaultdict d = defaultdict(True) 

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To answer the second part, a dict-comprehension is just what you need:

{k: k for k in range(10)} 

You probably shouldn't do this but you could also create a subclass of dict which works somewhat like a defaultdict if you override __missing__:

>>> class KeyDict(dict): ...    def __missing__(self, key): ...       #self[key] = key  # Maybe add this also? ...       return key ...  >>> d = KeyDict() >>> d[1] 1 >>> d[2] 2 >>> d[3] 3 >>> print(d) {}